1. FIELD OF INVENTION:
The present invention relates to receiving and tracking phase modulated signals.
2. DESCRIPTION OF PRIOR ART:
Prior art phase modulation receivers have in the past used various forms of tracking loops, such as phaselocked loops, squaring tracking loops and Costas loops to track the incoming signal carrier frequency. In each of these tracking loops, a voltage controlled oscillator (VCO) was driven through a loop filter by an error signal derived from a multiplication product of two signals. In phase-locked loops and squaring tracking loops, the output of the VCO was multiplied with the incoming signal to form the error signal. In the Costas loop, two error signals were formed. The first Costas loop error signal was formed by multiplying the VCO output, termed an I signal, with the incoming signal. The second Costas loop error signal was formed by multiplying a 90.degree. phase shifted I signal, termed a Q signal, with the incoming signal. Examples of such tracking loop receivers are in U.S. Pat. Nos. 3,289,082; 3,546,703; 3,550,131; 3,701,948; 3,710,257; and 3,736,507.
Receivers using these tracking loops were, however, limited to quadriphase or lower (e.g., biphase) modulation and to digital data. Also, these receivers used only the error signal portion of the incoming signal in tracking.